netdev - Re: [PATCH] net: Expand headroom to send fragmented packets in bridge fragment forward

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Message-ID: <20250411105450.GA395307@horms.kernel.org>
Date: Fri, 11 Apr 2025 11:54:50 +0100
From: Simon Horman <horms@...nel.org>
To: Huajian Yang <huajianyang@...micro.com>
Cc: pablo@...filter.org, kadlec@...filter.org, razor@...ckwall.org,
	idosch@...dia.com, davem@...emloft.net, dsahern@...nel.org,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	netfilter-devel@...r.kernel.org, coreteam@...filter.org,
	bridge@...ts.linux.dev, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: Expand headroom to send fragmented packets in
 bridge fragment forward

On Wed, Apr 09, 2025 at 03:44:44PM +0800, Huajian Yang wrote:
> The config NF_CONNTRACK_BRIDGE will change the way fragments are processed.
> Bridge does not know that it is a fragmented packet and forwards it
> directly, after NF_CONNTRACK_BRIDGE is enabled, function nf_br_ip_fragment
> will check and fraglist this packet.
> 
> Some network devices that would not able to ping large packet under bridge,
> but large packet ping is successful if not enable NF_CONNTRACK_BRIDGE.
> 
> In function nf_br_ip_fragment, checking the headroom before sending is
> undoubted, but it is unreasonable to directly drop skb with insufficient
> headroom.
> 
> Using skb_copy_expand to expand the headroom of skb instead of dropping
> it.
> 
> Signed-off-by: Huajian Yang <huajianyang@...micro.com>
> ---
>  net/bridge/netfilter/nf_conntrack_bridge.c | 14 ++++++++++++--
>  net/ipv6/netfilter.c                       | 14 ++++++++++++--
>  2 files changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/net/bridge/netfilter/nf_conntrack_bridge.c b/net/bridge/netfilter/nf_conntrack_bridge.c
> index 816bb0fde718..b8fb81a49377 100644
> --- a/net/bridge/netfilter/nf_conntrack_bridge.c
> +++ b/net/bridge/netfilter/nf_conntrack_bridge.c

...

> @@ -97,6 +97,16 @@ static int nf_br_ip_fragment(struct net *net, struct sock *sk,
>  
>  		return err;
>  	}
> +
> +expand_headroom:
> +	struct sk_buff *expand_skb;

Please move this declaration to the top of the function.

Flagged by W=1 builds with gcc 14.2.0 and clang 20.1.2.

> +
> +	expand_skb = skb_copy_expand(skb, ll_rs, skb_tailroom(skb), GFP_ATOMIC);
> +	if (unlikely(!expand_skb))
> +		goto blackhole;
> +	kfree_skb(skb);
> +	skb = expand_skb;
> +
>  slow_path:
>  	/* This is a linearized skbuff, the original geometry is lost for us.
>  	 * This may also be a clone skbuff, we could preserve the geometry for

...